1. Field of the Invention
The invention pertains generally to lithographic processes for producing devices such as, e.g., semiconductor devices and, more particularly, to sensitizing baths for chalcogenide resists.
2. Art Background
Lithographic processes play an important role in the manufacture of devices such as, e.g., semiconductor devices. During the manufacture of these devices, lithographic processes are used to pattern substrates, such as silicon wafers or processed silicon wafers which are, for example, wholly or partially covered by metal, silicon dioxide, or polysilicon. That is, a substrate is coated with an energy sensitive material called a resist. Selected portions of the resist are exposed to a form of energy which induces a change in the solubility of the exposed portions in relation to a given developing agent or etchant. The more soluble portions of the resist are removed and portions of the substrate are bared by applying the developing agent or etchant to the resist. The bared portions of the substrate are then treated, e.g., are etched or metallized.
Both organic and inorganic materials have been used as resists for the patterning of substrates. Exemplary inorganic materials are chalcogenide glass based materials, i.e., materials exhibiting a noncrystalline structure and whose major constituent is sulfur, selenium, or tellurium, or compounds thereof. Included among the chalcogenide glass based materials used as resists are germanium-selenium (Ge-Se) glass layers supporting relatively thin layers of silver selenide (Ag.sub.2 Se). Typically, a Ge-Se glass layer is evaporated or rf-sputtered onto the surface of a substrate. A thin layer of Ag.sub.2 Se is formed on the surface of the Ge-Se glass by, for example, dipping the glass into a Ag.sup.+ -containing aqueous solution, or a [Ag(CN).sub.2 ].sup.- - containing aqueous solution, the resulting chemical reactions with the glass yielding the Ag.sub.2 Se.
When an Ag.sub.2 Se-covered Ge-Se glass is exposed to an appropriate form of energy, silver ions from the Ag.sub.2 Se migrate into the exposed regions of the Ge-Se glass, decreasing the solubility of these regions to specific developers. This migration of silver ions, which is referred to as silver photodoping, is induced, for example, by UV light (in the wavelength range from about 2000 to about 4500 Angstroms), low energy electron beams (having energies ranging from about 1 keV to about 3 keV), and low energy ion beams (including ions such as helium, nitrogen, argon, xenon, and gallium ions with energies ranging from about 10 keV to about 30 keV). It is believed that silver photodoping involves electron-hole pair generation within the Ge-Se through, for example, the absorption of photons. The electric field produced by the electron-hole pair generation accelerates the diffusion of silver ions downwardly from the Ag.sub.2 Se layer into the Ge-Se, with the holes moving upwardly from the Ge-Se into the Ag.sub.2 Se to provide charge compensation.
After a Ge-Se resist is exposed to energy, the Se and Ag.sub.2 Se remaining on the surface of the resist is removed. This is done, for example, by immersion in a KI-I.sub.2 solution which dissolves, i.e., oxidizes, the Ag.sub.2 Se and Se to form SeO.sub.3.sup.2- and AgI.sub.4.sup.3-. Thereafter, the Ge-Se film is either dry developed, or wet developed. (See, for examle, R. G. Vadimsky, K. L. Tai, Abstract No. 318, 158th Electrochemical Society Meeting, Hollywood, Fla., October 5-10, 1980, regarding wet development of Ge-Se resists).
One of the advantages of Ge-Se resists in their high absorption cross-section for various forms of energy, including UV light, low-energy electron beams, and low-energy ion beams. For example, at least 60 percent of UV light, when incident on Ge-Se resists, is absorbed within a thin image layer about 100 to 300 Angstroms (.ANG.) thick. Consequently, good line width control (variations in line width are less than or equal to about 10 percent for linewidths of, for example, 1 .mu.m) is achieved with Ge-Se resists because little or no UV light penetrates beyond the thin image layer, and thus reflections from the substrates supporting the Ge-Se resists, with their attendant degradation in line width control, are avoided.
Another advantage of Ge-Se resists is their ability to resolve feature sizes smaller than 1 .mu.m while simultaneously achieving good line width control. It is believed that these properties are due to the so-called edge-sharpening effect. (Tai et al, Abstract No. 319, 158th Electrochemical Society Meeting, Hollywood, Fla., Oct. 5-10, 1980; and Tai et al, "Submicron Optical Lithography Using an Inorganic Resist-Polymer Bilevel Scheme," Journal of Vacuum Science Technology, 17(5), September-October 1980, pages 1169-1175, have explained this desirable effect).
While Ge-Se resists do exhibit the advantageous properties described above, their lithographic performance is critically dependent on the thickness, thickness uniformity, and morphology of the overlying Ag.sub.2 Se films. For example, the thickness of an Ag.sub.2 Se layer should be greater than about 50 .ANG. but less than about 150 .ANG., and should preferably be about 100 .ANG.. A thickness less than about 50 .ANG. is undesirable because such a film contains an inadequate amount of silver to adequately silver photodope the underlying Ge-Se glass. On the other hand, a thickness greater than about 150 .ANG. is undesirable because such a layer of Ag.sub.2 Se absorbs so much incident radiation that relatively few electron-hole pairs are formed within the Ge-Se glass, again resulting in an inadequate silver photodoping of the Ge-Se.
Not only should the Ag.sub.2 Se layer have a thickness ranging from about 50 to about 150 .ANG., but the thickness of the Ag.sub.2 Se layer should be substantially uniform (variations in thickness should be less than about 5 percent of the average thickness). Significant variations in thickness (more than about 5 percent) result in undesirably large variations in the amount of silver photodoped into the Ge-Se glass, producing degraded linewidth control.
Finally, the Ag.sub.2 Se layer should have relatively few pinholes (fewer than about 20/.mu.m.sup.2) and the pinholes, if present, should be relatively small (should have diameters smaller than about 0.05 .mu.m). A large number of pinholes (more than about 20/.mu.m.sup.2) and/or pinholes which are relatively large (having diameters larger than about 0.05 .mu.m), results in undesirably large variations in the amount of silver photodoped into the underlying Ge-Se glass.
In order to be useful in a production environment, the Ge-Se resists must exhibit reproducibly good lithographic performance. Thus, the sensitizing baths must not only yield Ag.sub.2 Se layers whose thicknesses range from about 50 to about 150 .ANG., but whose thicknesses are uniform (any variations in the thickness of an Ag.sub.2 Se layer should be less than about 5 percent of the average thickness), reproducible (the difference between the average thickness of any Ag.sub.2 Se layer and the desired thickness should be less than about 5 percent of the desired thickness), and have relatively few pinholes (fewer than about 20/.mu.m.sup.2) of relatively small size (having diameters smaller than about 0.05 .mu.m). Moreover, such Ag.sub.2 Se layers should be formed in less than about 20 minutes, in order for the baths to be economical.
While the Ag.sup.+ -containing aqueous baths, and the [Ag(CN).sub.2 ].sup.- -containing aqueous baths, do produce useful Ag.sub.2 Se layers, the latter baths are toxic and exhibit significant dip time sensitivity, i.e., small differences in dip time result in undesirably large differences in the thicknesses of the Ag.sub.2 Se layers (a difference in dip time of, for example, 10 percent yields a thickness difference greater than about 5 percent). In addition, both the former and latter baths intermittently produce Ag.sub.2 Se layers whose thicknesses are non-uniform (the layers exhibit thickness variations greater than about 5 percent of the average thickness). Finally, both the former and latter baths often yield Ag.sub.2 Se layers having an undesirably large number of pinholes (more than about 20/.mu.m.sup.2) and/or undesirably large pinholes (having diameters larger than about 0.05 .mu.m). (The addition of excess CN.sup.- ions to the [Ag(CN).sub.2 ].sup.- -containing aqeuous baths, as described in U.S. Pat. No. 4,343,887 issued to Heller et al on Aug. 10, 1982, does, however, significantly improve the performance of the [Ag(CN).sub.2 ].sup.- -containing baths.)
Thus, those involved in the development of chalcogenide resists have long sought, thus far without success, nontoxic sensitizing baths which yield silver composition-containing layers, e.g., Ag.sub.2 Se layers, whose thicknesses are substantially uniform (any variation in the thickness of a Ag.sub.2 Se layer is less than about 5 percent of the average thickness), insensitive to dip time (a difference in dip time of about 10 percent yields a thickness difference less than about 5 percent), and which have relatively few pinholes (fewer than about 20/.mu.m.sup.2) of relatively small size (having diameters smaller than about 0.05 .mu.m).